1. Field of the Invention
The present invention relates to a wavelength-division multiplexed self-healing passive optical network, which is capable of detecting cut-off and deterioration of feeder fiber and distribution fiber and restoring the network.
2. Description of the Related Art
A wavelength-division multiplexed (WDM) passive optical network (PON) can be configured to provide secure, secret communication. The communication capacity can be enlarged as required by subscribers. In addition, the number of subscribers can be increased by increasing the number of wavelengths to be allocated to new subscribers.
In general, the wavelength-division multiplexed passive optical network uses a double star structure where a central office (CO) is connected up to remote nodes (RN) instatled in the neighbor areas of subscribers by means of one feeder fiber and the neighbor areas are connected up to the subscribers by means of individual distribution fibers.
Multiplexed downstream signals are transmitted to the remote nodes through one feeder fiber, de-multiplexed by multiplexers/de-multiplexers located at the remote nodes, and then transmitted to the subscribers through the distribution fibers connected individually to the subscriber units.
Upstream signals output from the subscriber units are transmitted to the remote nodes through the distribution fibers connected individually to the subscriber units, multiplexed by multiplexers/de-multiplexers located at the remote nodes, and then transmitted to the central office.
In such a wavelength division multiplexed passive optical network, however, if unexpected accidents such as cut-off and deterioration of the feeder fiber or distribution fibers take place, this may result in loss of a mass of transmitted data even if the term of the accidents is shortened. Therefore, to minimize such a loss, the accidents are quickly detected so that the feeder fiber or distribution fibers are quickly restored.
So, there is a keen need for a wavelength division multiplexed self-healing passive optical network, which is capable of detecting accidents such as cut-off and deterioration of the feeder fiber or distribution fibers on an implemented optical network and restoring the optical network.
FIGS. 1a and 1b show examples of a conventional wavelength division multiplexed self-healing ring network.
In general, a ring network in which nodes are connected together in the form of a ring is used as the topology for restoring a wavelength division multiplexed optical network smoothly when an accident such as cut-off of a fiber takes place.
In such a conventional ring network, a central office 100, remote node 1 (denoted “remote node 200”) and remote node 2 (denoted “remote node 300”) are connected together by two fibers which comprise a composite feeder fiber.
The two fibers consist of a working fiber and a protection fiber. In the normal state, the central office 100 multiplexes by means of its components 101, 102, 103 signals having various wavelengths (for example, λ1 and λ2) to form two identical optical signals and transmits each signal over the respective one of the two fibers. Unidirectional add/drop multiplexer 108, 109, 112, 113 selectively drops an incoming optical signal to the respective one of the optical switching devices 110, 111, 114, 115 and replaces the dropped signal by adding from that switching device another signal having the same wavelength(s).
Similarly, remote node 200, 300 transmits the same two optical signals over respective ones of the two fibers to the central office 100 which de-multiplexes incoming optical signals by means of de-multiplexers 106, 107. Likewise, selection from among the two optical signals is made using optical switching devices 104, 105. When an accident takes place such as cut-off of the working fiber, as shown in FIG. 1b, the conventional ring network performs a self-healing operation as follows. Assuming, for example, that the working fiber of the composite feeder fiber between remote nodes 200, 300 is cut off, since remote node 300 cannot receive channel 2 (λ2) traveling counterclockwise over the working fiber, it receives channel 2 (i.e., λ2) traveling clockwise over the protection fiber. In addition, since remote node 200 cannot add and transmit channel 1 (λ1) in the counterclockwise direction over the working fiber, it transmits channel 1 (λ1) in the clockwise direction over the protection fiber by switching the optical switching device 110. In this way, the conventional ring network is capable of healing the accident occurring in the network.
Although it is efficient to use the conventional self-healing ring network when the central office is distant from a plurality of remote nodes to some degree (several tens of km), a structure of the ring network is not suitable for a passive optical network where the central office is connected in close proximity to the remote nodes and the remote nodes are connected to each other. Namely, since the passive optical network commonly has a star structure, there is a need for a new self-healing method different from the self-healing method for the ring network.